An electric power steering apparatus (EPS), which is equipped with a motor control unit and applies a steering assist force (assist force) to a steering mechanism of a vehicle by a rotational force of a motor, applies a driving force of the motor as the steering assist force to a steering shaft or rack shaft by a transmission mechanism such as gears, a belt or the like through a reducer. Such a conventional electric power steering apparatus performs a feedback control of a motor current to accurately generate the torque of the steering assist force. The feedback control adjusts the voltage applied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by the adjustment of a duty of a PWM (Pulse Width Modulation) control.
A general configuration of the electric power steering apparatus (EPS) will be described with reference to FIG. 1. A column shaft 2 (a steering shaft, a handle shaft) of a handle (steering wheel) 1 is connected to steered wheels 8L and 8R via reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5 and tie rods 6a and 6b and further through hub units 7a and 7b. Moreover, the column shaft 2 is provided with a torque sensor 10 for detecting the steering torque Th of the handle 1, and a motor 20 for assisting the steering force of the handle 1 is connected to the column shaft 2 through the reduction gears 3. Toa control unit (ECU) 30 for controlling the electric power steering apparatus, an electric power is supplied from a battery 13 as a power supply, and an ignition key signal is inputted via an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command based on a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12 and controls an electric current supplied to the motor 20 with a voltage control value Vref obtained by performing compensation or the like on the calculated current command value. A steering angle sensor 14 is not indispensable and may not be disposed, and the steering angle can be obtained from a rotational sensor connected to the motor 20.
The control unit 30 is connected to a controller area network (CAN) 40 for giving and receiving various information on the vehicle, and the vehicle speed Vel can also be received from the CAN 40. Moreover, the control unit 30 is also connected to a non-CAN 41 for giving and receiving communications other than the CAN 40, analog/digital signals, radio waves and so on.
In this electric power steering apparatus, the control unit 30 is configured mainly with a control section of an MCU (including a CPU, an MCU or the like), and general functions performed by a program in the control section are configured, for example, as shown in FIG. 2.
The functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Th from the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 are inputted into a current command value calculating section 31, and the current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Th and the vehicle speed Vel by using an assist map or the like. In an adding section 32A, the calculated current command value Iref 1 is added to a compensation signal CM from a compensating section 34 for improving the characteristics. In a current limiting section 33, the maximum value of the added current command value Iref2 is limited. The current command value Irefm with the limited maximum value is inputted into a subtracting section 32B and subtracted by a detected motor current value Im.
The subtraction result I (=Irefm−Im) in the subtracting section 32B is proportional-integral (PI)-controlled by a PI-control section 35. The PI-controlled voltage control value Vref is inputted into a PWM-control section 36 together with a modulation signal (a carrier) CF, and a duty is calculated. The motor 20 is PWM-driven through an inverter 37 by a PWM signal for which the duty is calculated. The motor current value Im of the motor 20 is detected by a motor current detecting means 38 and inputted into the subtracting section 32B for the feedback.
The compensating section 34 adds a detected or estimated self-aligning torque (SAT) to an inertia compensation value 342 in an adding section 344, further adds a convergence control value 341 to the addition result in an adding section 345, and inputs the addition result as the compensation signal CM in the adding section 32A to improve the characteristics.
When the motor 20 is a three-phase brushless motor, the details of the PWM-control section 36 and the inverter 37 are configured, for example, as shown in FIG. 3. The PWM-control section 36 comprises a duty calculating section 36A and a gate driving section 36B. The duty calculating section 36A calculates PWM-duty values D1 to D6 for three phases by the voltage control value Vref in accordance with a predetermined equation. The gate driving section 36B drives gates of FETs serving as driving elements, by the PWM-duty values D1 to D6 as well as compensates dead time and turns ON/OFF the FETs. The modulation signal (the carrier) CF is inputted into the duty calculating section 36A, and the duty calculating section 36A calculates the PWM-duty values D1 to D6 in synchronization with the modulation signal CF. The inverter 37 comprises three-phase bridges of the FETs, and each FET is turned ON/OFF by the PWM-duty values D1 to D6 to drive the motor 20.
As well, a motor release switch 23 is interposed between the inverter 37 and the motor 20 to block the supply of electric current when the assist control is stopped or the like. The motor release switch 23 comprises FETs with a parasitic diode inserted in each phase.
For this electric power steering apparatus, in recent years, vehicles equipped with idle stop and sailing functions or the like have been put in practical use for the purposes of saving fuel consumption and reducing emissions (wastes). In these vehicles, a starter for cranking upon the restart of the engine generally consumes considerable electric power. Thus, especially when the vehicles are driven in an urban area and the like where the vehicles stop and start frequently repeatedly, the battery being the power supply is consumed tremendously as the frequency of starting the engine increases. In this type of idle stop vehicles, the operations of the electric power steering apparatus and the fan of the air conditioner are continued even during the automatic engine stop, and these controls are also factors to accelerate the consumption of the battery. Accordingly, a phenomenon where the battery voltage temporarily decreases greatly at the moment of the engine cranking occurs. Then, the power supply reset of the ECU and the MCU occurs, and there has been a trouble losing the learning contents and so on up to that time point.
Further, in addition to cope with the low voltage caused by the increase in the frequency of starting the engine as described above, a demand for the continuation functions of the control sections such as the ECU and the MCU at the low voltage is becoming very high for safety reasons.
As a solution to these problems, an idle stop vehicle of Japanese Unexamined Patent Publication No. 2002-38984 A (Patent Document 1) has been proposed. The idle stop vehicle disclosed in the Patent Document 1 comprises a voltage compensation controller and a voltage compensation circuit. In a normal engine operation, the voltage compensation controller opens both a discharging switch and charging switch of the voltage compensation circuit. When an engine stop signal is inputted, the voltage compensation controller closes the charging switch, and the electric current from a battery is supplied to a capacitor of the voltage compensation circuit to charge the capacitor. When the start is operated in accordance with the satisfaction of the engine starting conditions and an input voltage is lower than a first set value upon the starter operation due to the consumption of the battery, the voltage compensation controller opens the charging switch in the voltage compensation circuit and closes the discharging switch to discharge the electric charge accumulated in the capacitor. When the input voltage from the battery becomes a second set value abnormality, the voltage compensation controller opens the discharging switch to return to the initial state. In this way, the capacitor is charged before the engine start, and the voltage compensation circuit is operated in synchronization with the engine start.
Moreover, in addition to the contents of the above Patent Document 1, a driving voltage supply device of Japanese Unexamined Patent Publication No. 2005-256643 A (Patent Document 2) employs a driving voltage supply method which controls the supply of a driving voltage to electric loads in consideration of heat generation of the latter part, monitors the voltage of a battery constantly and boosts the voltage by a control unit and a boosting circuit when the voltage is lower than below the minimum voltage. A device of Japanese Unexamined Patent Publication No. 2009-255742 A (Patent Document 3) immediately determines the state of the battery upon a starter driving, reduces an assist force of an actuator of a mechanism of an electric power steering.
Furthermore, a vehicle motor control unit of Japanese Unexamined Patent Publication No. 2013-224097 A (Patent Document 4) has a relay switch provided between a battery and a capacitor of a motor driving circuit, and the relay is turned off for a predetermined period of time when a return (starter start) signal from the idling stop is received. When the battery becomes low voltage, a path for discharging from a capacitor to a battery is blocked, and power is supplied to a control section via a power supply path from the capacitor to the control section to prevent the reset of the control section. When the relay is turned on after a predetermined period of time elapses, the motor driving control is immediately started, thereby shortening the time taken from the engine restart to the motor driving.